Metal-air electrochemical cells utilize oxygen from ambient air as a reactant in an electrochemical reaction to provide a relatively lightweight power supply. Generally described, a metal-air cell includes an air-permeable cathode and a metallic anode separated by an aqueous electrolyte. During operation of a zinc-air cell, for example, oxygen from the ambient air is converted at the cathode to hydroxide ions, zinc is oxidized at the anode and reacts with hydroxide ions, and water and electrons are released to provide electrical energy.
Cells that are useful for only a single discharge cycle are called primary cells, and cells that are rechargeable and useful for multiple discharge cycles are called secondary cells. An electrically rechargeable metal-air cell is recharged by applying voltage between the anode and the cathode of the cell and reversing the electrochemical reaction. During recharging, the cell discharges oxygen to the atmosphere through the air-permeable cathode and the anode is electrolytically reformed by reducing to the base metal, the metal oxides formed during discharge.
One problem with metal-air cells is that the difference between the ambient relative humidity and the internal relative humidity of the cell can cause the metal-air cell to fail. If ambient humidity is greater than the equilibrium relative humidity value for the metal-air cell, the metal-air cell will absorb water from the air through the cathode and fail due to a condition called flooding. Flooding may cause the cell to leak. If the ambient humidity is less than the equilibrium relative humidity value for the metal-air cell, the cell will release water vapor from the electrolyte through the air cathode and fail due to drying out. In most environments where a metal-air battery cell is used, failure occurs from drying out.
Drying out and flooding are greater problems for secondary (rechargeable) metal-air cells than for primary metal-air cells. Although ambient humidity may not be a sufficient problem to flood or dry out a cell after a single cycle, cumulative water gain or loss from a series of discharge and charge cycles can cause premature failure of a secondary metal-air cell.
Thus, it has been desirable to control the exposure of the air cathode in a metal-air cell to air so that the amount of oxygen supplied to the cathode is sufficient to generate the power demands of the cell, but the amount of air to which the cathode is exposed is insufficient to cause premature failure of the cell through flooding or drying out. To control the exposure of the air cathode in a metal-air cell to air, cell case structures and air management systems have been developed to limit the air exposure of the air cathode to air.
One approach to controlling the water vapor content of a metal-air cell is through the use of an air manager system. In an air manger system, a fan supplies air through a system of sized openings and plenums in a housing containing an array of metal-air cells. The exposure of the air cathode of the cell to air is controlled by the rate of delivery of air by the fan and the size of the plenums and the openings in the housing. Such arrangements have been effective to control the water content of metal-air cells; however, they are relatively costly to produce because of the necessity of air-tight seals and appropriate part tolerances to deliver the appropriate amount of air to the cells.
Another method for controlling the amount of water vapor in a metal-air cell is with a membrane such as a thin layer of microporous material disposed adjacent an air cathode. The pores in the membrane control the exposure of the air cathode to air to limit the flow of water vapor in and out of the cell.
Still another article for controlling the water content of a metal-air cell is a perforated mask covering the air cathode and forming an air plenum adjacent the air cathode. The openings in the mask are sized and distributed across the mask so as to allow a sufficient amount of air to the air cathode for an adequate production of power from the cell, but limit the amount of air to which the air cathode is exposed so as to prevent premature failure of the cell from flooding, drying out, or contamination. Although such arrangements are effective to control the water vapor content of the cell to some extent, greater control is desirable.
Accordingly, there is a need for a metal-air cell with enhanced control over the water content of the cell.